The cell envelope constitutes a thick layer of highly insoluble protein on the inside of the plasma membrane of cornfield epidermal cells and of other stratified squamous epithelial cells. Of several putative protein components, none has hitherto proven to be a major component because of major differences in amino acid composition, levels of mRNA or of expressed protein. We have recently identified a new protein, termed loricrin, that fulfills all of the requirements of a major constituent of the cell envelope. Loricrin clones were first identified in a mouse epidermis, and now in more detail, from human epidermis. Loricrins are glycine-rich proteins that contain the highly flexible glycine loop motif. They are crosslinked in cell envelopes by isodipeptide N epsilon- (gamma-glutamyl)lysine bonds. By use of Northern slot blotting techniques and mathematical modeling estimates, we have determined that loricrin constitutes up to 70-80% of the total mass of mature cell envelopes of mouse and human epidermis, while other components are much less than this. We postulate that the cell envelope is first assembled by addition of certain soluble proteins such as involucrin, cystatin A, etc., followed by massive subsequent deposition of loricrin.